Oscar Joseph Slater (8 January 1872 – 31 January 1948) was the victim of a notorious miscarriage of justice in Scotland. Wrongly convicted of murder and sentenced to death, he was freed after almost two decades of hard labour at Scotland’s HM Prison Peterhead through the efforts of multiple journalists, lawyers, and writers, including Sherlock Holmes author Sir Arthur Conan Doyle.

He was born Oskar Josef Leschziner in Oppeln, Upper Silesia, Germany, to a Jewish family. Around 1893, possibly to evade military service, he moved to London, where he purportedly worked as a bookmaker using various names, including Anderson, before settling on Slater for official purposes. He was prosecuted for alleged malicious wounding in 1896 and assault in 1897 but was acquitted in both cases.

In 1899, Slater moved to Edinburgh and by 1901 was living in Glasgow. He was known to be a well-dressed dandy, who billed himself variously as a dentist and a dealer in precious stones, but was believed to earn his living as a gambler.

In December 1908, Marion Gilchrist, aged 83 years, was beaten to death in a robbery at West Princes Street, Glasgow, after her maid, Helen Lambie, had popped out for ten minutes. Although she had jewellery worth £3,000 (equivalent to £400,000 in 2023) hidden in her wardrobe, the robber, who was disturbed by a neighbour, had rifled through Mrs. Gilchrist's personal papers and taken only a brooch. Slater left for New York five days after the murder and came under suspicion, as apparently before the murder, a caller to Gilchrist's house had been looking for someone called "Anderson", and Slater had coincidentally previously been seen trying to sell a pawn ticket for a brooch.

The police soon realised that the pawn ticket was for an entirely different brooch and a false lead, but notwithstanding the contradictory evidence, still applied for Slater's extradition. While Slater was advised that the application would probably fail anyway, he voluntarily returned to Scotland to clear his name of the alleged crime.

At his trial presided over by Lord Guthrie, whose summing up was highly prejudicial, defence witnesses provided Slater with an alibi and confirmed that he had announced his trip to America long before the date of Mrs. Gilchrist's murder. He was convicted by a majority of nine to six (five "not proven" and one "not guilty"). In May 1909, he was sentenced to death, with the execution to take place before the end of that month. However, Slater's lawyers organised a petition that was signed by 20,000 people, and the Secretary of State for Scotland, Lord Pentland, subsequently issued a conditional pardon and commuted the sentence to life imprisonment. Slater would serve nineteen years at Peterhead Prison.

The following year, the Scottish lawyer and amateur criminologist William Roughead published his Trial of Oscar Slater, highlighting flaws in the prosecution. The circumstantial evidence against Slater included his alleged "flight from justice". The prosecution's evidence and witnesses identifying Slater as a suspect, including maid Helen Lambie, were also criticized as fleeting and otherwise unreliable, prejudiced, tainted, or coached. In particular, Slater was conspicuously contrasted with nine off-duty policemen in a rigged identification parade.

Slater received little support from within Glasgow's Jewish community, which was attributed towards concerns around drawing attention to Slater's Jewish identity in light of the case's notoriety and the potential for a rise in antisemitism as a result.